Land remediation helps future-proof neighbourhoods

Land being redeveloped for new housing by Kāinga Ora's urban development team is being put through a rigorous remediation process that involves investigating the impact of previous human use, and returning the land to a state that is safe and durable for construction.

Piritahi’s Lean Phuah, who leads the land remediation team, says that land is remediated for two main reasons: firstly, to remove or encapsulate the impacted soil and secondly, to make the land durable and safe for future construction workers and residents.

“Although Piritahi is working with land that has generally been used for state housing only, all of Auckland’s land has been impacted by human use,” says Phuah. Building practices and materials used in the past have had an impact on the soil. For example, lead particles may be found in the soil where houses have been painted with lead-based paint.

Levels of lead in the soil vary from property to property, says Phuah. One of the reasons for such differences is related to differing levels of maintenance that have occurred across individual properties. For example, properties with higher levels of lead may have been painted more frequently, or painters may have been less diligent about removing their sanding dust.

Another source of contaminants is building or domestic waste. A common practice in the past was to throw the waste material generated from building houses (offcuts) into the trenches which form the foundations. Uncontrolled filling and disposal of domestic waste has also occurred in some areas. These materials are identified during Piritahi’s investigation phase and then safely dealt with so that future users of the site do not get exposed to them.

Piritahi is also finding low levels of asbestos in some of the soil being tested. Traces of asbestos could stem from rainwater running off asbestos roofs, cladding and fencing over time. However, “not all soil is scraped back because it has been impacted,” says Phuah. “The land sometimes has to have the topsoil removed for geotechnical purposes.” Topsoil will consolidate and settle over time which can cause shifting, warping and cracking of building materials, so it’s often not suitable as a base for construction.

From as little as 100mm to as much as half a metre of soil may be scraped off the surface and removed from the site, or impacted soil is encapsulated and covered with clean soil. Clean topsoil is also added back at the end of the building process in areas where there is to be landscaping.

Phuah explains that Piritahi has developed a robust approach to land remediation in the developments it's helping Kāinga Ora to deliver. “We're making sure that both workers developing the land and future users and occupiers of the land are not exposed to impacted soil, and that runoff from the site entering the stormwater system doesn’t negatively affect the environment”.

After the scraped soil is removed to an approved landfill, the newly revealed ground is tested again and must meet an acceptable level, in line with the national environmental standards set by the government.